Malignant transformation by the avian rous Sarcoma Virus (RSV) is induced by a virus-coded protein which plays no role in virus reproduction. Attempts have been made to identify the transforming protein of the Bryan "high titer" strain of RSV (RSV-BH) and to determine its biochemical function in transformation. Characteristic morphological and metabolic features of REV-BH transformed cells have been used as a basis for examining possible functions of the transforming protein, and a mutant of RSV-BH which induces temperature-dependent transformation has been useful in distinguishing between direct and indirect effects of the protein. Mammalian cells from several species have been transformed by RSV-BH, and these are being used to analyze viral transformation in the absence of inherent complicating features of avian host cells.